Piston



July 4, 1950 G. L. MOORE 2,513,814

PIs'roN Filed Jan. 21, 1947 2 Sheets-Sheet l llllllllll George L. M00 eJuly 4, 1950 G. L. Moons PISTON Filed Jan. 21. 1947 ffy 2 Sheets-Sheet 2INVENTR.

Patented July 4, 1950 PISTON George L. Moore, Hudson, Ohio, assignor toAluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania Application January 21, 1947, Serial No. 723,344

2 Claims.

This invention relates to pistons for internal combustion engines or forlike purposes and is principally concerned with problems attendant uponthe use of a piston material, such as magnesium or aluminum alloy, whichis relatively expansible in the sense that it has a higher coeiiicientof expansion than the material of the cylinder in which the piston isdesigned to operate.

The object of this invention is to provide a piston, particularly apiston having a skirt of oval cross section, which may be fitted to asatisfactory clearance in the cylinder when cold, which, when heated inoperation, will not expand sufiiciently to produce excessive -scufiingor friction, but which has a skirt portion forming an effectiveoperational seal with the cylinder wall and serving to center the pistonduring its operation in the cylinder. Further objects of the inventioninclude the provision of pistons which will satisfactorily perform suchfunctions without the necessity of including in the piston structurecontrol members or struts made of material different in coeiiicient ofthermal expansion than the material of the piston body or of slotting orotherwise breaking the continuity of the lower portion of the pistonskirt to control the expansion thereof or the flow of heat therein.

The piston of this invention has a skirt which is, in large part, of theconventional oval shape such as is usually produced by a cam grindingoperation. The major axis of the oval shape defined by the outersurfaces of the skirt is substantially at right angles to the axis ofthe wrist pin bosses and may be of constant length or of progressivelydecreasing or increasing length from top to bottom of the skirt if it isdesired, in accordance with well known principles, to provide a taperingskirt. In accordance with this invention, however, the minor axis of theoval shape increases abruptly in length at a point below the wrist pinbosses of the piston so that, in effect, the surface of the skirtportion of the piston denes two oval shapes which, having a common majoraxis, blend in the thrust faces of the skirt and gradually separate inthe area under the wrist pin bosses. Thus under each wrist pin boss thelower portion of the skirt forms a raised area which blends into theshape of the upper skirt in the region of the thrust faces.

The invention, its purposes and objects may be more readily describedand explained with reference to specific embodiments thereof such asshown in the accompanying drawings in which:

- conventionally located in the skirt 2.

Fig. 1 is an elevation view of a piston constructed in accordance withthis invention;

Fig. 2 is an elevational view taken at right angles to the view in Fig.1;

Fig. 3 is a diagram on which is projected the vvarious mentioned axes ofthe piston shape, the

divergences in` dimensions being greatly exaggerated for purposes ofillustration.

Fig. 4 is an elevational view of another type of piston constructed inaccordance with this invention.

Fig. 5 is an elevational view taken at right angles to the View in Fig.4.

The piston shown in Figs.1 and 2 of the drawing is, except as indicated,of conventional shape and includes a head portion I to which is appendeda skirt 2. Transverse slots 3 separate the thrust faces 4 of the skirtfrom the head portion l. Further slots, an example of which is thedownwardly extending slot 5 indicated in dotted line in Fig. 2, may alsobe provided. Wrist pin bosses, openings of which are shown at 6, are

Conventional grooves 1 are provided in the head portion I for thereception of piston rings. The piston is formed, as by cam grinding,with a skirt portion the surfaces of which form an oval shape. The majoraxis of this oval lies, as indicated in Fig. 3, at right angles to theaxis of the wrist pin bosses and extends between the thrust faces 4. Inthe piston of this invention the major axis is of the same lengththroughout the height of the skirt unless, as above mentioned, the skirtis tapered, in which case the major axis gradually increases in lengthfrom top to bottom or bottom to top, as the case may be, of the pistonskirt. Throughout the greater portion of the height of the piston skirtthe shorter or minor axis, which is disposed at right angles to themajor axis, as indicated in Fig. 3, likewise is of constant length, orof constantly increasing length if a skirt taper is provided, but, inaccordance with this invention, the minor axis` abruptly increasesinlength at a point below the wrist pin bosses so that the lower portionof the skirt defines an oval of greater minor axis than the upperportion of the skirt. Preferably this lower portion of the skirt is notgreat in height and lies well below the wrist pin bosses.

Thus the skirt of the piston shown in Figs. 1 and 2 is fashioned todefine a lower oval and an upper oval shape which form a juncture at apoint below the wrist pin bosses and which differ only in the increasedlength of the minor axis of the lower shape thus resulting in theformation of a. step, ledge or shoulder 8 (see Figs. 1 and`2) below thewrist pin boss openings, which ledge gradually disappears in thedirection of the thrust faces 4 as the two oval shapes blend together toconform to the same major axis. To achieve the objects of this inventionthe divergence in length between the minor axis of the lower ovate shapeand the upper ovate shape need not be large, a divergence in the orderof .004 to .006 inch usually being suflicient in passenger car sizes ofpistons.

In operation the lower skirt portions below the shoulders or offsets 8assume, under operational temperatures, a shape closely approximatingthat.

of the cylinder in which the piston operates, thus forming an effectiveseal between the bottom portion of the `skirt and the cylinder wall,over a surface which while great enough in area to effect this purposeand to prevent sidewise motion or tilting of the piston in the cylinder,is yet suiciently small to avoid excessive friction with the cylinderwall.

A piston having a skirt of the shape described may be formed in anysuitable manner. A convenient method is to grind a conventionally castpiston skirt to form the two ovate shapes.

It is also possible, if desired, to so cast the piston as to roughlyform offset portions below the wrist pin boss opening which may later befinally shaped and dimensioned in the grinding operation. Such a pistonis illustrated in Figs. 4 and 5. In this piston, which comprises a headportion I to which is appended a skirt II, the thrust faces I2 of theskirt are separated from the head portion I0 by transverse slots I3. Aslot I4 extends downwardly in one of the thrust faces. Wrist pin bosses,the openings of which are shown at I5, are located in the skirt I I.Conventional grooves I6 are provided in the head portion I0 forreception of piston rings. The

vbottom edge of skirt I I is arcuate in shape under each wrist pinopening I so that the length of the skirt reaches a maximum through thethrust face portion and a minimum through the boss carrying portion. Arelief portion I8, formed in the piston at the.t1me of casting, extendsaround each wrist pin opening and downwardly to within a short distancefrom the edge of the skirt when it terminates, thereby forming a step,ledge or shoulder I9 below the wrist pin openings I5 which ledge blendsinto the thrust faces The piston is cam ground so that the skirt I2.portion is oval in shape, the major axis of the oval lying at rightangles to the axis of the wrist pin bosses and extending between thrustfaces I2. This major axis is of the same length throughout the height ofthe skirt unless the skirt be tapered, in which case the major axisgradually increases in length from top to bottom or bottom to top, asthe case may be, of the piston skirt. The minor axis, which is at rightangles to the major axis, abruptly increases in length at the shoulderor ledge I9 below the wrist pin openings. so that the lower portion ofthe skirt defines an oval of greater minor axis than the ovate portionof the skirt above the shoulder I9. In operation the piston performs inthe manner described with reference to the piston illustrated in Figs. 1and 2.

Among the advantages to be derived from the use of the principles ofthis invention is the fact that the described objects and results may beobtained without resort to inserts, control members or struts made ofmaterial different from the alloy which forms the body of the pi.- ton,thus allowing the use of a ,completely homogeneous piston. Also thelower portion of the skirt need not, as is the case in some otherpistons designed to achieve similar objects, be slotted or otherwisebroken in continuity in order to provide means by which excesscircumferential expansion may be absorbed.

This application is a continuation in part of my copending applicationSerial No. 522,999, filed February 19, 1944, now abandoned.

I claim: y

1. An internal combustion engine piston formed of a relativelyexpansible alloy and comprising a head, a skirt depending from said headand wrist pin bosses disposed in said skirt; the surfaces of the skirtdefining two ovate shapes having a common major axis at right angles to`the axis of the wrist pin bosses, one of said ovate shapes having alonger minor axis than the other ovate-shape and being defined by thelower portion of the skirt and joining the ovate shape of lesser minoraxis at a point below said wrist pin bosses, the change from said lesserto said longer minor axis being abrupt at said point and the respectivelengths of said lesser and said longer minor axes being such as toprevent operational contact between the cylinder wall and-` said' pistonskirt over that portion of said ovate shape of lesser minor axis whichincludes and* is adjacent to the termini of said minor axes.`

2. An internal formed of a relatively expansible alloy and comprising ahead, a skirt depending from said head and wrist pin bosses disposed insaid skirt; the surfaces of the skirt defining two ovate shapeshaving-'a common major axis at right angles to the axis of the wrist pinbosses, one of said ovate shapes having a longer minor axis than theother ovate shape and being defined by the lower portion of the skirtand joining the ovate shape of lesser minor axis at a point below saidwrist pin bosses, the change from the longer minor axis of the lowerovate shape to the lesser minor axis of the upper ovate shape beingabrupt, whereby the lower ovate shape forms a stepped portion under saidwrist pin bosses, which portion merges gradually into the thrust facesof the piston, the respective lengths of said lesser and said longerminor axes being such as to prevent operational contact between thecylinder wall and said piston skirt over that portion of said ovateshape of lesser minor axis which includes and is adjacent to the terminiof said minor axes.

GEORGE L. MOORE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name v Date 1,623,328 Austin Apr. 5, 19272,044,074 Jardine June 16, 1936 2,046,789 Norton July '7, 1936 2,262,132Berry Nov. 11, 1941 combustion engine pistonv l

